Exosomes are cell-derived vesicles of about 30 to 120 nm associated with a wide range of biological fluids. Although their biological functions are not yet fully characterized, their potential utility in various diagnostic and therapeutic applications has been considered. They are commonly purified by techniques that discriminate on the basis of size, such as ultracentrifugation, flow cytometry, filtration, size exclusion chromatography, and field flow fractionation (Kang, D., et al J. Proteome Res., 7 (2008) 3475-3480; Lasser, C., et al, J. Vis. Exp. 59 (2012) e3037; Sokolova, V., et al, Colloids Surf. B: Biointerfaces, 87 (2011) 146-150).
The composition of exosomes is discussed by A. Vlassov et al (Biochim. Biophys. Acta 1820 (2012) 940-948). They contain a nucleotide and protein cargo, consisting of an estimated less than 10,000 nucleotide units and less than 300 proteins. These are enclosed within a lipid-bilayer membrane that is variously enriched, depending on the source of the exosomes, with cholesterol, ceramides, sphingolipids, and phosphoglycerides with long saturated fatty acyl chains. The outer surface also bears saccharide groups including mannose, polylactosamine, alpha-2,6-sialic acid, and complex N-linked glycans.
Glycine is a naturally occurring amino acid that exists in zwitterionic form at physiological pH. It is used frequently in the field of exosome research at a concentration of 100 mM to quench unreacted amino-reactive aldehydes on solid particles following covalent linkage of exosomes to such particles (Vincent-Schneider, H., et al, Intl. Immunol., 14 (2002) 713-722). It has also been described as bulking agent in formulations of purified exosomes (H. Lamparski et al, U.S. Pat. No. 6,812,023 B1, 2004). Glycine is known to promote non-specific interactions between proteins and hydrophobic surfaces (Gagnon, P., J. Chromatog. A. 1221 (2012) 57-70; Muller, E., et al, J. Sep. Sci., 36 (2013) 1327-1334). The high lipid content of cell membranes is understood to mediate elevated surface hydrophobicity.